Suppression mechanism by a thin interface W oxide layer is investigated for intermetallic reaction at Al/W interfaces. The interface reaction scheme of Al/W was analyzed by in situ x-ray photoemission spectroscopy for the interface formation and subsequent annealing in ultrahigh vacuum. Without the W oxide layer at the interface, annealing at temperatures higher than 450 degrees C leads to an intermetallic reaction forming the bimetallic compound WAl12 through the entire metal layer, while no reaction between Al and W occurs at room temperature. The intermetallic reaction causes the lowering of W 4f core level by 0.7 eV. This is explained by the donation of a lone electron pair from an Al 3s orbital to a vacant W 6s orbital in bimetallic bonding. The interface thin W oxide layer, composed of WO3, is consumed in the oxidation-reduction reaction with metallic Al. The reaction product Al2O3 inhibits the intermetallic reaction. This results in an increase in annealing temperatures to as high as 600 degrees C and longer annealing time to form WAl12.